1. Field Of The Invention
This invention relates to a method and apparatus for detecting mercury amalgamation of aluminum, and more particularly relates to a method and apparatus for detecting the presence of mercury amalgams in the aluminum components of gas processing equipment.
2. Description of The Prior Art
The material of choice for heat exchangers in LNG and LPG plants is aluminum. Aluminum is chosen primarily because of its high thermal conductivity, ease of fabrication, reasonable cost and availability. A difficulty arising from the use of aluminum to construct heat exchangers for LNG plants is the fact that all natural gas contains measurable amount of mercury. Mercury is hazardous to aluminum equipment because the mercury promotes rapid deterioration of the aluminum once the mercury penetrates the coating of aluminum oxide usually present on aluminum surfaces. The aluminum becomes embrittled and is subject to failure when tensile stresses are applied.
In co-pending application, Ser. No. 093,277, filed on Sept. 4, 1987, now U.S. Pat. No. 4,380,829, which herein by reference, there is described a method for restoring the ductility of aluminum tubing whose metallurgical properties have been damaged by exposure to mercury. Thus, such a method relates to treating the aluminum body after the damage has occurred rather than detecting mercury amalgamation before the aluminum has deteriorated.
There are currently three ways of detecting mercury contamination of aluminum bodies when such contamination is suspected. Such conventional measures include visual observation on freely exposed surfaces, x-ray examination of hidden surfaces, as within crevices, and in the case of cracks, the use of conventional ultrasonic devices.
When visual observation is employed, the greatest reliance is placed on the sighting of aluminum corrosion product, variously termed, powder, whiskers, feathers or some similarly descriptive term for the crystals of beta-aluminum trihydrate, which is the characteristic result of moist air on amalgamated aluminum. However, the powder is so ephemeral and blows away so easily that it cannot be relied upon as a true indication of the site of the mercury corrosion attack.
X-rays have been used to locate incidental mercury liquid in the form of droplets, when such droplets are trapped in crevices, as in the space between a backing ring used to make a weld, and the weld itself. However, the sensitivity of x-rays towards amalgamated surfaces is not sufficiently high to detect mercury when it occurs in this form.
Ultrasonic devices have been employed to locate cracks caused by mercury, but such devices cannot discriminate between cracks caused by some other source, such as shrinkage or porosity, and those cracks caused by mercury liquid metal embrittlement.